Mechanism for linux output in terminal when using system function read() & write()

Question

Code:

#include <stdio.h>
#include <unistd.h>

int main(void)
{
    char buf[BUFSIZ];
    int n;

    while((n = read(0, buf, BUFSIZ)) > 0 && printf("1:%d ", n))
    {
        printf("2:%d ", n);
        write(1, buf, n);
    }

    return 0;
}

pupu(my input)
pupu(output)
popopo(my input)
popopo(output)
1:5 2:5 1:7 2:7(output)

My question: How does it work?

(why buffer text output before n_read?)

Answer 1

The standard I/O functions (like printf ) are buffered , meaning output to stdout isn't printed until its buffer is full or explicitly flushed.

On the other hand, writing directly to the output file descriptor is not buffered and is written directly.

What you have here is you mixing direct and buffered output, and the buffered output isn't actually written until the program exits and the buffer is flushed.